Strategies for back contact engineering in high-performance flexible kesterite solar cells

Abstract

Kesterite solar cells are pivotal in advancing flexible photovoltaic devices integrated into buildings and products. High-purity Mo foil is one of the most promising flexible substrates, thanks to its outstanding properties. However, the kesterite/Mo foil interface is extremely reactive and chemically unstable during the high-temperature selenization process required to obtain the kesterite crystalline phase, forming a thick MoSe2 layer. The role of MoSe2 in kesterite solar cells is still under discussion, as it can affect the charge extraction at the back contact and the kesterite grain growth. This work reports on the functionalisation of Mo foil to fabricate flexible kesterite solar cells based on Li-doped and Ag-alloyed Cu2ZnSn(S,Se)4 (Li-ACZTSSe) films grown using the molecular ink method. MoS2, Al2O3, MoO2, and MoO3 were inserted between the precursor layer and the substrate to investigate their impact on the MoSe2 thickness, the morphology and composition of the absorber, interface chemistry, carrier collection at the back contact, and the related photovoltaic parameters. It is demonstrated that MoO3 as an interlayer significant enhances device performance by improving the absorber quality and back contact, achieving an efficiency of 11.2% with a 15 µm thick MoSe2 layer. To the best of our knowledge, this is the first report demonstrating that an over-thick MoSe2 layer is not significantly detrimental to the performance of flexible kesterite-based devices when the Li-ACZTSSe crystallinity and grain growth are improved and its decomposition at the back is prevented.

Postprint (published version)